It is commonly known and accepted that cellulose fibers can be mechanically reduced to essentially separate fibers by several physical means, such as pulverizing, shredding, grinding and the like. Several novel uses have been found for these ground fibers as a raw material in such products (but not limited to) as roofing coatings, textured paints, sound deadeners, driveway sealants, and the like. Fibers used in these applications are semi-dispersed, but usually their total separation is not required.
Another use which is rapidly gaining acceptance is the incorporation of a low level of fibers in asphaltic products used in the construction of highways. Asphalt-aggregate mixtures have been used worldwide for many years as road pavements, because of a plentiful supply and relatively low cost. Over the years, a number of modifications have been made in manufacturing and installation techniques, such as the addition of binding materials, various gradations of sand, gravel and the like. Despite these improvements, the trade is constantly seeking ways to improve the longevity of the product.
An extensive evaluation of highways in place in several foreign countries by a panel of construction engineers from the United States in 1990, revealed the use of a product, primarily in Europe, which provides a dramatic increase in the performance of asphalt pavements. It is called Stone Mastic Asphalt, also known as SMA. In general terms, an SMA is a gap-graded aggregate mix design containing approximately 6% asphalt and an asphalt modifier. The modifier is typically either a polymer or fiber. European roads using these mixes have performed very favorably.
The fibers, at a 0.3% level, exert a thixotropic or thickening on the asphalt, and stabilize the asphalt in the mix. This allows an acceptable viscosity to be maintained with a higher concentration of asphalt than could otherwise be used.
There are inherent problems in introducing loose fibers into a drum or batch plant. The low density of the fibers causes them to remain airborne without special equipment modifications. The present state of the art in Europe is to circumvent this problem by utilizing a pelleted product consisting of 50% cellulose fibers intimately mixed with 50% bitumen (asphalt), see U.S. Pat. No. 5 028 266. This allows for controlled addition of the fibers by adding the preformed pellet of substantial density into the hot mix, where it theoretically melts and uniformly disperses. This practice apparently works adequately; however, an inherent cost is obvious in the preparation and transportation of the pelleted mixture.